The present invention is directed to a carbon fiber reinforced carbon composite (C/C composite). More particularly, the present invention is directed to the C/C composite with impregnated and coated pyrolytic carbon.
The size of a pulling single crystal apparatus for the Czochralski (CZ) method has seen a recent trend of increasing with increasing diameter of the single crystal. The structural elements for use under a high temperature atmosphere have been made of a high-density and a high-purity isotropic graphite with high mechanical properties under high temperature. The size of the graphite elements is also increasing with the size of the CZ apparatus. This produces a handling problem caused by the increased weight of the existing graphite elements and a problem of reduction in effective processing size of the inside of the apparatus.
The C/C composite has many excellent properties of lightweight and strong mechanical strength, as compared with the graphite material. By virtue of these properties, even when reduced in thickness, the structural elements of the C/C composite can have a strength equal to that of the graphite material, to enable its effective use for a processing chamber of the apparatus. In addition, by virtue of being lightweight, good handling can be achieved in, for example, placement in the apparatus. By virtue of these properties, the crucible components used in the CZ apparatus having a large diameter are now moving from those made of graphite to those made of a C/C composite.
However, the C/C composite has a large specific surface area and a large diameter of pores. Accordingly, the C/C composite reacts with SiO gas in the apparatus easily and the reacting rate to silicon carbide (SiC) is faster than that of graphite.
The inventors have proposed a C/C composite with impregnated and coated pyrolytic carbon (PyC) by CVI (Chemical Vapor Infiltration) for a crucible of the CZ apparatus in PCT WO97-49844 in order to prevent reacting with SiO gas. In the C/C composite with impregnated and coated PyC by CVI, the impregnated PyC layer is formed deep, but the coated PyC layer is formed thin. Consequently, the C/C composite can not prevent reaction with SiO gas and formation of SiC. In the C/C composite with impregnated and coated PyC by only CVD (Chemical Vapor Deposition), the coated PyC layer is formed thick, but the impregnated PyC layer is formed shallow. Consequently, the C/C composite can prevent reaction with SiO gas, but when the coated PyC on the surface is worn out, the C/C composite is going to change to SiC due to the impregnated PyC layer being thin. Accordingly, the C/C composite with impregnated and coated PyC by CVD has safety problems when applied to the pulling single crystal apparatus.
It is the objective of the present invention to provide a C/C composite with high resistance to reacting to form SiC, by impregnating and coating PyC by CVI and CVD continuously, and a pulling single crystal apparatus of the C/C composite.
The present invention therefore provides a C/C composite, obtained by a method comprising: impregnating a pitch or a resin into a molded member formed of carbon fibers for densification thereof; forming an impregnated PyC layer by CVI in a furnace after densification; and forming a coated PyC layer continuously on the impregnated PyC layer by CVD without taking the C/C composite out from the furnace after CVI. The molded member can be subjected to high purification under a halogen gas atmosphere at 2,000 to 2,500xc2x0 C. before CVI. A density difference between the impregnated layer and the coated layer of PyC of the C/C composite is not more than 0.2 g/cm3, and more preferably 0.1 g/cm3. A thickness of the coated PyC layer is 10 to 100 xcexcm, more preferably 20 to 80 xcexcm. An average surface roughness of the coated PYC layer is not more than 5 xcexcm.
The surfaces of the CVD coated PyC can be formed very smooth due to gaps in the carbon fibers on the surface being filled with the impregnated PyC by CVI. Due to the density difference between the impregnated layer and the coated layer of PyC of the C/C composite being not more than 0.2 g/cm3, and more preferably 0.1 g/cm3, the resistance to delaminating of the PyC coated layer increases due to the disappearance of a definite boundary between the impregnated layer and the coated layer. Due to the thickness of the coated PyC layer being 10 to 100 xcexcm, more preferably 20 to 80 xcexcm, SiO gas can be prevented from penetrating into the inside of the C/C composite. Furthermore, due to the average surface roughness of the coated PyC layer being not more than 5 xcexcm, the C/C composite can be prevented from reacting to form SiC, in view of a decrease in the contact surface area with SiO gas.
The impregnated PyC layer and the coated PyC layer are formed continuously by CVD after CVI in the same furnace without removal from the furnace after CVI. Accordingly, a clear boundary between the impregnated PyC layer and the coated PyC layer does not appear, and there are no impurities in the boundary of the impregnated PyC layer and the coated PyC layer due to never exposing in the air.
The present invention therefore provides a pulling single crystal apparatus made from a high purity C/C composite with high resistance to reacting to form SiC by the impregnated and coated PyC by CVI and CVD continuously.